1. Field of the Invention
The present invention relates to gas turbine engines, and more specifically to a system to control a blade tip clearance in the turbine section.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
A gas turbine engine such as those used in industrial application for electric power production and those used in aerospace for aircraft propulsion operate at improved efficiency if the blade tip clearance is kept to a minimum to prevent leakage of the gas stream past the blades. The blade tip clearance gap is not a continuous gap length during engine operation. The gap length changes due to different thermal growth coefficients between the rotor disk and the engine casing securing the outer shroud that forms the gap between the blade tips. During engine start-up, the stator (casing) will heat up faster and therefore will thermally expand faster than will the rotor shaft. The blade tip clearance gap will decrease during engine start-up. During steady-state full load operation, the gap will remain within a narrow range. During engine shut-down, the stator will shrink faster than the rotor disk, and therefore the blade tips will rub against the shroud casing. This rub during shut-down is a very undesirable situation because it causes the blade tips and the outer shroud segments to wear prematurely. In the case of other previously disclosed arrangements, although it has been possible to keep the thermally induced relative movements of the components low for many operating states by selecting suitable pairs of materials, it is also true there that an optimum rotor blade tip gap in each case prevails only in a specific steady state. In every other state, less beneficial conditions again occur.
Prior art techniques make use of abradable seals that will allow for rub without causing too much damage. However, abradable seals are an added expense to the engine. One such reference is U.S. Pat. No. 4,177,004 issued to Riedmiller et al on Dec. 4, 1979 which discloses a COMBINED TURBINE SHROUD AND VANE SUPPORT STRUCTURE in which a single turbo machine support structure provides both close tolerance radial positioning of a turbine shroud and close tolerance axial positioning of a vane. Radial movement of the vane without attendant radial movement of the shroud is accommodated by the use of surfaces designed to minimize constraint between the vane and the support structure. The linkage is further provided with cooling means to isolate thermal variations proximate the vane end thereof from the support structure end thereof.
Some of the known prior art systems and methods of controlling the blade tip clearance are those like U.S. Pat. No. 2,762,559 issued to Faught on Sep. 11, 1956 which discloses an AXIAL FLOW COMPRESSOR WITH AXIALLY ADJUSTABLE ROTOR. The rotor shaft carrying the blades that form the tip clearance is adjusted in an axial direction by a fluid powered cylinder based upon the engine RPM and/or the compressor outlet temperature. Because the blade tips and the shroud that forms the tip clearance are conical shaped, axial displacement of the rotor with respect to the stationary shroud will change the tip clearance.
U.S. Pat. No. 6,676,372 issued to Scholz et al. on Jan. 13, 2004 discloses a GAS TURBINE WITH AXIALLY MUTUALLY DISPLACEABLE GUIDE PARTS in which the shroud casing is axially displaced instead of the turbine blades via the rotor shaft as in the above Faught patent.
U.S. Pat. No. 5,330,320 issued to Mansson on Jul. 19, 1994 entitled METHOD AND A DEVICE IN ROTATING MACHINE discloses another method of controlling blade tip clearance in a gas turbine engine by axially displacing the rotor shaft. A piston is used to displace the rotor in the axial direction. When the machine has become heated after a start or after a load increase, the piston 9 is caused to be extended whereby the compressor part 4 with the rotor shaft 3 and the turbine disc 2 is moved towards the interior of the stator cone 16 and the clearance is reduced. The operation of the piston 9, for control of the blade tip clearance, can be performed either manually or automatically. Extension of the piston 9 may, for example, take place after a certain period of time after a start or when a certain power has been attained. Shortening of the piston 9 may, for example, take place in connection with a stop impulse being given to the machine.
U.S. Pat. No. 4,149,826 issued to Torstenfelt on Apr. 17, 1979 entitled GAS TURBINE discloses a gas turbine having a rotor and a stator, the stator defining a blade channel which is of generally conical configuration and being axially movable relative to the rotor. A thermal responsive means which is heated in accordance with the temperature of the gas in the turbine moves the stator and rotor axially relative to each other to vary the gap there between so as to maintain the clearance between the rotor blades and the stator housing within predetermined values. The gap is subject also to the rotational forces acting on the rotor blades.
U.S. Pat. No. 5,203,673 issued to Evans on Apr. 20, 1993 entitled TIP CLEARANCE CONTROL APPARATUS FOR A TURBO-MACHINE BLADE discloses an apparatus is provided for controlling rotating blade tip clearance in a turbo-machine utilizing conical tipped blades. The apparatus comprises an approximately conical blade ring mounted for axially sliding displacement in the turbo-machine cylinder. The conical blade ring encircles the tips of the rotating blades and forms a blade tip clearance there between. The tip clearance is controlled during operation of the turbo-machine by axially displacing the conical blade ring. Piston cylinders, actuated by pressurized fluid extracted from the turbo-machine, are used to displace the blade ring. Springs, adapted to bias the blade ring into a position of increased tip clearance, oppose the piston cylinder so that failure of the piston cylinder will not result in a loss of tip clearance. A blade tip clearance sensor transmits information on the tip clearance to a controller that automatically adjusts the blade ring axial position to continuously maintain the optimum tip clearance by regulating the pressure of the fluid supplied to the piston cylinders.
U.S. Pat. No. 6,273,671 issued to Ress, Jr. on Aug. 14, 2001 entitled BLADE CLEARANCE CONTROL FOR TURBOMACHINERY discloses a system that includes a gas turbine engine having a shroud and a rotor with one or more blades. The rotor rotates within the shroud to pressurize a fluid during operation of the engine. An electromagnetic actuator is also included that is operable to move the shroud relative to the rotor to adjust clearance between the shroud and blades. In addition, a controller is included in this system to determine a desired amount of clearance in accordance with an operating mode of the engine. The controller generates an actuation signal to change the clearance in correspondence with the desired amount. The electromagnetic actuator responds to the actuation signal to provide the desired amount of clearance.
None of the above mentioned references discloses a system to control blade tip clearance in a gas turbine engine in which the rotor is axially displaced due to pressure differences acting on the rotor from engine operations as in the present invention as disclosed below.
It is an object of the present invention to provide for a gas turbine engine with blade tip clearance control that is a function of an operating characteristic of the engine such as a pressure differential acting on a rotor disc.
It is another object of the present invention to provide for a gas turbine engine with blade tip clearance control for both the turbine blades and the compressor blades of the engine.